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Sexual selection is the theory that competition for mates between
individuals of the same sex drives the evolution of certain traits. It is distinct from ecological selection which is the competition for food within the species' ecological niche. Many traits,
e.g. smooth skin or fur, strong muscles, fluid motions, appear not only to enable hunting or
gathering but also to be important sexual attractors, especially in the more intelligent species. For these, ecological and sexual selection both operate on a trait.
Ambiguous combinations of both types of selection acting on the same traits is
usually referred to as natural selection. Some accounts refer to
natural selection as strictly ecological and as distinct from sexual, but this appears to be a holdover from Victorian sexual prudery, and further fails to distinguish combinations
of the two "natural" processes from other concepts of evolution, e.g. evolution of societies or moral evolution.
Traits amenable to sexual selection, which give an individual an advantage over its rivals in courtship, without being
directly involved in reproduction, are called secondary sex characteristics. Sex differences
directly related to reproduction and serving no direct purpose in courtship are called primary sex characteristics.
In most sexual species (the seahorse is a notable exception), the males and females have different equilibrium strategies, due to a difference in time investment: the male
can potentially spend five or ten minutes impregnating the female, and have a chance at producing offspring. The female, by
comparison, may spend as long as a year pregnant (and unable to produce other
children during that time). A male is also much less able than a female to be
certain about whether or not he is the true parent of a child, and so will be less interested in spending his energy helping a
child who may or may not be related to him. As a result of these factors, males are much more willing to mate than females, and
so females are the main ones doing the choosing (except in cases of rape, which occurs in
certain primate species). This causes sexual selection often to be more pronounced in
males than in females: for example, the peacock has elaborate and colorful tail
feathers which the peahen lacks.
The peacock's tail is also an example of a trait that, despite decreasing the apparent biological fitness of the organism, still persists. Sexually selected
traits tend to become exaggerated: if most females are looking for long-tailed males, then each female individually does better
to select a long-tailed male, since then her male children are more likely to succeed. (Of course, the females do not actually
have this thought process; this kind of decision is simply an evolutionarily stable strategy.)
An example of sexual selection in human evolutionary history is humans' hairlessness
relative to the other great apes. This is part of a general physiological resemblance between adult humans and adolescent
chimpanzees (adult humans resemble young chimpanzees to a greater extent than
they resemble young humans or adult chimpanzees). This youthful appearance may have evolved because males prefer young-looking
mates (a young female is more likely to survive pregnancy). Blond hair lasting into
adulthood is another example of a trait that makes a human look younger.
The theory of sexual selection was first proposed by Charles Darwin
in his book The Origin
of Species. A later book, The Descent of Man, and Selection in Relation to Sex dealt with the
subject exhaustively.
The sciences of evolutionary psychology and
sociobiology study the influence of sexual selection in humans, though this
is often a controversial field. The field of epigenetics is broadly concerned
with the competence of adult organisms within a given sexual, social, and ecological niche, which includes the development of
mating competences, e.g. by mimicking adult behavior.
K- and r-selection
One of the most famous types of sexual selection is selection by number of offspring.
Some animals, like human (sexually mature after adolescence) and Northern Gannet (5-6 years),
produce few offspring. Others reproduce quickly, but unless raised in an artificial environment, most offspring do not survive to
adults. A rabbit (mature after 8 months)
produces 10 - 30 offsprings per year, a Nile Crocodile (15 years) produce 50, and fruit fly (10-14
days) produce up to 900. Both strategies can be favoured by evolution: animals
with few offspring can time nurturing and protecting them, hence greatly decreasing the need to reproduce; on the other hand,
animals with many offspring do not need to spend parental energy on nurturing, allowing more energy to be devoted to survival and
more breeding.
These two strategies are known as K-selection (few offspring) and r-selection (many
offspring). (The letters "r" and "K" derive from the names used in the mathematical formulae in the original theory). Which strategy is favoured depends on a wide range of
circumstances.
See also: courtship
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